In the course of rewinding a yarn from a delivery bobbin to a winding bobbin in a bobbin winding machine, the traveling yarn is continuously monitored for yarn defects. The yarn defects are cut out of the yarn, the two yarn ends being created are connected with each other again, and the winding process is started again. In the course of thusly removing yarn defects, it must be assured that the yarn defect is completely removed, regardless of whether it involves punctiform or long thin or thick sections. Furthermore, only the defective yarn portion is to be removed during the process, so that the least amount of yarn waste is created.
A winding device is known from German Published, Non-Examined Patent Application DE-OS 20 36 898, wherein the winding bobbin is immediately braked after a yarn defect has been detected, without the yarn being cut. The rotational pulses of the bobbin or the winding drum are counted from the appearance of the yarn defect until the winding bobbin comes to a stop. As the winding bobbin is subsequently unwound, the rotational pulses are counted backward to zero when the defective yarn is unwound. The unwound yarn is aspirated as a loop and then contains the defect. After cutting off the defective yarn loop, the resulting defect-free upper thread and lower thread are connected with each other.
This known method is only sensible at low winding speeds, because otherwise a considerable length of yarn would be wound onto the winding bobbin from the appearance of the defect until the stopping of the winding bobbin, which then constitutes waste. Furthermore, when braking sharply there is the danger of the yarn tearing. If the surface of the bobbin is braked by means of a winding roller, there is the danger of damage to the outermost yarn layers. The time required for unwinding the excess yarn furthermore results in a reduction in the efficiency of the machine.
It is also known from German Patent Publication DE 39 11 505 A1 to permit the traveling yarn to run up on the sharply braked winding bobbin following the appearance of a defect. This method also results in a considerable amount of waste yarn being wound onto the winding bobbin in the braking phase. The time required for the reverse winding of the winding bobbin during unwinding of the defective yarn is either limited by a time relay, wherein the predetermined unwinding time is based on experimental values or, in the course of unwinding, the yarn is directed past a sensor for detecting the passage of the defective portion. The steps for eliminating the defective portion and for restoring the yarn connection are initiated thereafter.
If the time for unwinding the defective yarn portion is limited by a time relay, the unwinding time must be set to the most disadvantageous circumstances which could result when the yarn end wound on the winding bobbin is not immediately aspirated and the defect is an extended one. For this reason, the suction tube is left to aspirate along the circumference of the delivery bobbin rotating in the unwinding direction until it can be assumed on the basis of experimental values that the yarn end has been aspirated. As a result, completely uncontrollable yarn lengths are aspirated into the suction tube, depending on whether the yarn start was already found immediately upon positioning the suction tube in front of the delivery bobbin, or the starting end of the yarn was detected only at the end of the predetermined search time.
A method for cleaning out yarn defects is known from European Patent Publication EP 0 419 821 B1, wherein a differentiation is made between conventional yarn defects and yarn defects caused by an auxiliary piecing yarn which is used to repair yarn breaks in ring spinning machines. If a yarn defect caused by the normal spinning process occurs, a short, predetermined yarn piece is pulled off the winding bobbin and removed, while when a defect caused by piecing occurs, which can be detected by means of the yarn cross section, sufficient yarn is always removed from the winding bobbin so that the auxiliary yarn in its entire known length, together with the pieced location, is removed. Therefore two different yarn lengths are predetermined for the removal of any given defect, the shorter predetermined yarn length being intended for defects caused by the spinning process, and the longer predetermined yarn length being intended for defects caused by the auxiliary yarn. Because of the fixed yarn lengths to be removed, this method does not permit the precision removal of short yarn defects caused by spinning because then it would not be assured that longer yarn defects would be completely removed because of the fixed cleaning out length.